Technical Field
The present invention relates to an ethylene-vinyl alcohol copolymer resin composition and a method for producing the same.
Background Art
Ethylene-vinyl alcohol copolymers (hereinafter, may be also referred to as “EVOH(s)”) are superior in oxygen barrier properties, transparency, oil resistance, antistatic properties, mechanical strength and the like, and have been broadly used as various types of packaging materials such as films, sheets and containers, etc.
In general, superior appearance characteristics of e.g., the absence of coloring such as yellowing, and the absence of unevenness of hues are required for EVOH resin compositions containing EVOH. In addition, since the films and the like are usually formed by a melt molding method, EVOH resin compositions are expected to have superior appearance characteristics (absence of generation of gels and seeds (i.e., dirt under paint), absence of occurrence of coloring such as yellowing, and the like) without any quality unevenness even in melt molding, long-run workability (a property enabling a molded article to be obtained without change of physical properties such as viscosity, etc., and without occurrence of fish-eye, streak, etc., if molded for a long period of time), and the like. In addition, films, sheets and the like are often formed with a multilayer structure having an EVOH resin composition layer for the purpose of improving oxygen barrier properties and the like. When such a multilayer structure is to be obtained, a metal salt is often contained in an EVOH resin composition in order to improve adhesiveness between layers. However, it is known that when a metal salt is included in an EVOH resin composition, coloring such as yellowing is more likely to be caused, and thus appearance characteristics are deteriorated. Particularly, in use such as sheet molding, mold flash of the sheet after obtaining the molded article has been recovered and recycled; however, repeated recycling results in enhanced deterioration of the EVOH resin composition, thereby leading to disadvantages such as deteriorated appearance of the molded article due to an increase of defects such as gels and seeds.
Under such circumstances, in order to improve appearance characteristics of the EVOH resin composition, various proposals have been made such as a saponification product of a vinyl acetate-derived polymer obtained by polymerizing one or more types of monomer that includes vinyl acetate, followed by adding thereto a conjugated polyene compound having a boiling point of 20° C. or greater, and saponifying the vinyl acetate-derived polymer (Patent Document 1), an ethylene-vinyl ester copolymer saponification product composition containing 100 to 5,000 ppm of a hydroxycarboxylic acid and/or a salt thereof in terms of hydroxycarboxylic acid equivalent, 50 to 500 ppm of an alkali metal in terms of metal equivalent, and 20 to 200 ppm of an alkaline earth metal in terms of metal equivalent (Patent Document 2), and a resin composition for use in modifying a hydroxyl group-containing thermoplastic resin characterized by containing an ethylene-vinyl acetate copolymer saponification product, and 0.001 to 10% by weight of at least one type selected from a boric acid compound, phosphoric acid or a compound thereof and a fatty acid salt (Patent Document 3). However, these EVOH resin compositions can be still further improved with regard to appearance characteristics such as hues.
In addition, as an organic compound-based oxygen absorbent, or for the purpose of inhibiting degradation of EVOH in a film-forming solution containing a tertiary amine, blending a multivalent phenol into an EVOH resin composition has been known (Patent Document 4 and Patent Document 5). However, these EVOH resin compositions are disadvantageous in that when 0.1% by mass or more of the multivalent phenol is blended into EVOH in order to achieve the intended oxygen-absorbing effect or degradation inhibitory effect, the hue of the EVOH resin composition is deteriorated.